[aspiringphysician]

I have a question: When you add a methyl group to bromobenzene, why does it mostly add ortho- rather than para-? I know it has to do with resonance, but I drew resonance structures for both, and I can't figure out why one is better than the other. Plus, I figured because bromine is such a large molecule, the methyl group would rather add para- to avoid steric hindrance, but I guess I'm wrong. 

[Telamond]

Methyl is a Electron Donating Substituent on Phenyl, therefore it's meta/para directing.
Why it adds mostly ortho rather than para is because there are two ortho positions and only one para. Statistics.
Unless it's a really big bulky substituent like t-butyl the ortho will always be favored.

[Honclbrif]

Methyl is a Electron Donating Substituent on Phenyl, therefore it's meta/para directing

Methyl is o/p directing, and you don't count it until its on the ring. Br is running this show.

The problem with alkylating arenes is that the alkyl groups themselves are activators, so once you've got on on a second is more likely to add etc... and it turns into a mess. Usually less of a fuss to do F.C acylation followed by reduction, or a transition metal catalyzed coupling.

[fledarmus]

Which still doesn't answer the first question - why does Br direct the methyl preferentially to the ortho position?

My question is, under what conditions? I haven't been able to find any literature yet that gives the proportions of ortho and para products with a standard Friedel-Crafts reaction, no doubt because it is older than SciFinder catalogs reactions and I don't have access to Beilstein here. Not that I doubt the basis for your question, but I would like to know what ratio your example has, using what conditions.

AlCl₃ is a very activating Lewis acid for these reactions, and tends to give less control over substitution patterns - at that point it may well be the fact that there are two ortho positions and one para position that would dominate, as Telamond suggested (ignore the methyl confusion). Using other Lewis acids or other methyl donors might give different results.

[orgopete]

The Ortho-Para Ratio

An aromatic substitution reaction of a benzene derivative bearing on ortho, para-directing group would give twice as much ortho as para product if substitution were completely random, because there are two ortho positions and only one para position available for substitution. However, this situation is rarely observed in practice; it is often found that the para substitution product is the major one in the reaction mixture. In some cases this result can be explained by the spatial demands of the electrophile. For example, Friedel-Crafts acylation of toluene gives mainly all para substitution product and almost no ortho product. The electrophile cannot react at the ortho position without developing van der Waals repulsions with the methyl group that is already on the ring. Consequently, reaction occurs at the para position, where such repulsions cannot occur,

Typically, para substitution predominates over ortho substitution. but not always. For example, nitration of toluene gives twice as much o-nitrotoluene as p-nitrotoluene. This result occurs because the nitration of toluene at either the ortho or para position is so fast that it occurs on every encounter of the reagents; that is, the energy barrier for the reaction is insignificant. Hence, the product distribution corresponds simply to the relative probability of the reactions. Because the ratio of ortho to para positions is 2:1, the product distribution is 2:1. In fact, the ready availability of o-nitrotoluene makes it a good starting material for certain other ortho-substituted benzene derivatives.

In summary reasons for the ortho-para ratio vary from case to case and in some cases the reasons are not well understood. Whatever the reasons for the ortho-para ratio, if an electrophilic aromatic substitution reaction yields a mixture of ortho and para isomers, a problem with her separation arises that must be solved if the reaction is to be useful. Usually syntheses that give mixtures of isomers are avoided because, in many cases, isomers are difficult to separate. However, the ortho and para isomers obtained in many electrophilic aromatic substitution reactions have sufficiently different physical properties that they are really separated.

Loudon, G. Marc. "Organic Chemistry" 4th ed. 2005 (p. 722-723) This was from books.google.com, but it is no longer linked and I don't know the complete reference.